Are there null like thing in solidity

Question

    struct buyer{        uint amount;        Status status;     }      mapping(address=>buyer) public buyers;     mapping(uint=>address) buyerIndex;     uint public buyerNum;     //Order a product.     function(){       uint doubleValue=value*2;       uint amount=msg.value/doubleValue;        if(buyers[msg.sender]==null){ //Error in this line       buyer abuyer=buyer({amount:amount,status:Status.Created}); //Error in this line       buyerNum++;       buyerIndex[buyerNum]=msg.sender;       buyers[msg.sender]=abuyer;     }else{       buyers[msg.sender].amount+=amount;     }       Order(msg.sender,amount*doubleValue,amount);   } 

If a buyer is not recorded in the buyer mapping, then buyerNum++; but I don't know how to tell whether a buyer is in the mapping

Answer 1

In solidity every variable is set to 0 by default.

You should think of mappings as all possible combinations are set to 0 by default.

In your specific case I would use the following:

if (buyers[msg.sender].amount == 0)